3SUM

#include <bits/stdc++.h>
using namespace std;
using ll = long long;

const int N = 5000;
multiset<ll> S;
int T[N];

int main() {
  ll n, x;
  cin >> n >> x;
  for (int i = 0; i < n; ++i) {
    int k;
    cin >> k;
    T[i] = k;
    S.insert(k);
  }
  for (int i = 0; i < n; ++i) {
    S.erase(S.find(T[i]));
    for (int j = 0; j < i; ++j) {
      ll k = x - T[i] - T[j];
      if (S.count(k)) {
        int ind = i+1;
        while (T[ind] != k) ++ind;
        assert(T[i]+T[j]+T[ind] == x);
        cout << i+1 << ' ' << j+1 << ' ' << ind+1 << '\n';
        return 0;
      }
    }
  }
  cout << "IMPOSSIBLE\n";
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED